1. Field of the Invention
This invention relates to a method and equipment for reducing contaminants such as volatile organic compounds (VOC's) and carbon monoxide (CO) normally present in dryer offgas that is discharged into the atmosphere from a moist organic product drying process. The equipment includes a product dryer, recuperative thermal oxidizing apparatus, a furnace having a burner, which serves to deliver hot products of combustion to the thermal oxidizing apparatus, and a gas-to-gas heat exchanger of the indirect type having a hot gas side and a cool gas side, hereinafter referred to as the primary heat exchanger, for bringing the hot gaseous output from the thermal oxidizing apparatus that is ultimately discharged into the atmosphere into indirect heat exchange relationship with recycle dryer offgas to increase the temperature of the recycle dryer offgas prior to its reentry into the dryer.
Efficient thermal oxidation of VOC's and CO requires correlation of four factors occurring simultaneously:
1) Adequate temperature;
2) Adequate oxygen concentration;
3) Adequate residence time; and
4) Adequate turbulence.
In the present process, a non-preheated portion of the dryer offgas is directed to the burner, while another preheated portion of the dryer offgas that is removed from the stream thereof after passage through the primary heat exchanger is directed to the thermal oxidizing apparatus. The flow rates of the non-preheated portion of the dryer offgas and the preheated portion of the dryer offgas, and the input of fuel to the furnace are controlled and adjusted to provide a hot gaseous output from the thermal oxidizing apparatus that is at a temperature of at least about 1600° F. with an optimum 5% oxygen content by volume, which are sufficiently high to substantially oxidize VOC's and CO in dryer offgas that is discharged into the atmosphere. Introduction of the non-preheated portion of the dryer offgas into the burner also lowers the flame temperature thereby reducing the nitrogen plus oxygen based compounds (NOx's) of the hot products of combustion emanating from the furnace.
2. Description of the Prior Art
Dryers have been used for many years to lower the moisture content of a variety of organic products, such as grain, including distiller's grain and the like, which nominally may have a water content as high as 60-75%. The recent emergence of ethanol plants producing substantial quantities of moist distiller's grain as output residue requiring drying for further commercial use, has rekindled interest in more efficient drying processes while, at the same time, necessitating that dryer offgas discharged into the atmosphere contain reduced amounts of VOC's, CO and NOx's.
Commercial drying equipment has been previously designed and constructed to dry organic products to a predetermined acceptable level, which is normally about 10% moisture by weight, wet basis. It has been known for some time to incorporate thermal oxidizing apparatus in processes and equipment for drying moist organic products in order to lower the VOC and CO content of the product output from the dryer. For systems that utilize recuperative thermal oxidation processes (as opposed to end-of-pipe regenerative thermal oxidation processes) that are similar to one another, these processes have primarily involved the rudimentary steps of bypassing a non-preheated first portion of the dryer offgas to a mixing chamber interposed between a furnace and thermal oxidizing apparatus. A second portion of the offgas has been heat exchanged against the gaseous output from the thermal oxidizer, before being recycled back to the dryer.
In order to reduce the VOC and CO content of dryer offgas introduced into the atmosphere employing a thermal oxidizer, the hot gaseous output from the oxidizer should be at least about 1600° F. and the oxygen concentration should be at least about 5% by volume. Heretofore, the temperature of the output from the thermal oxidizer has been limited to temperatures in the order of 1400° F. when the oxygen concentration is increased to 5% by volume; hence, VOC and CO reduction has not been optimum.
Even though residence time of the offgas being oxidized was not restricted and gas turbulence not a significant factor, it was not heretofore feasible to adequately control both temperature of the thermal oxidizer, and its oxygen concentration, in order to significantly lower the VOC and CO content of the offgas introduced into the atmosphere. The temperature and the oxygen concentration could be controlled individually, but not simultaneously for most efficient operation of the thermal oxidizing apparatus.
FIGS. 1-3 in the drawings hereof are flow diagrams of representative prior art single-stage recuperative thermal oxidation processes where an effort, although only marginally successful, was made to reduce the VOC and CO content of dryer offgas discharged into the atmosphere. A required 1600° F. temperature of the hot gaseous output from the thermal oxidizer could not be obtained to minimize the VOC and CO content using any one of these prior processes.
In the prior art one-stage dryer offgas recuperative thermal oxidation processes of FIGS. 1-3, in each instance, moist organic material was introduced into a dryer with the resultant dried product exiting therefrom. A gaseous medium, consisting primarily of steam generated from the evaporation of product moisture and then superheated in the primary heat exchanger, was introduced into the dryer to reduce the moisture content of the product. The dryer offgas was separated into a first relatively cool portion, while the remaining portion was directed to the cool side of the primary heat exchanger. The preheated gaseous output from the cool side of the primary heat exchanger was then recycled back to the dryer.
The cool offgas portion was directed to a mixing chamber forming a part of conventional recuperative thermal oxidizing equipment that normally included a burner connected to a furnace that, in turn, was connected to a mixing chamber joined to a thermal oxidizer. Alternatively, a portion of the cool offgas directed to the mixing chamber could be redirected to the burner in the function of flue gas recycle for NOx reduction. Sources of natural gas fuel and combustion air were supplied to the burner. The hot gaseous output of the thermal oxidizer, after being directed through a tempering chamber, which reduced the temperature of the gaseous output, was introduced into the hot gas side of the primary heat exchanger. A portion of the hot gaseous output exiting from the hot side of the primary heat exchanger was returned to the tempering chamber, while the remainder of the hot gaseous output exiting from the hot side of the primary heat exchanger was discharged into the atmosphere.
In the system shown in FIG. 1, under the conditions representative of that process, the maximum temperature of the hot gaseous output from the thermal oxidizer was of the order of 1500° F. and the oxygen concentration in the thermal oxidizer was of the order of 2.8% by volume. A modification of the system shown in FIG. 1 is the one-stage system shown in FIG. 2, which provides a larger quantity of combustion air to the burner in an attempt to increase the oxygen content in the gaseous output of the thermal oxidizer. The one-stage system shown in FIG. 2, when operated under the representative conditions of that process, resulted in a thermal oxidizer output temperature of no more than about 1375° F. with an oxygen concentration in the gaseous output of the thermal oxidizer of the order of 5.0% by volume. A modification of the system shown in FIG. 2 is the one-stage system shown in FIG. 3, which adds a combustion air heater in an attempt to increase the outlet temperature of the thermal oxidizer. The one-stage system shown in FIG. 3, when operated under the representative conditions of that process, produced a thermal oxidizer output temperature that did not exceed about 1430° F. with an oxygen concentration in the gaseous output of the thermal oxidizer of the order of 5.0% by volume. Although the improvements described for the systems shown in FIGS. 2 and 3 increased the thermal oxidizer oxygen concentration to the requisite 5.0% by volume, all of these prior art systems had thermal oxidizer output temperatures well below the desirable level of 1600° F. regardless of oxygen concentration.